This invention relates to an electronic musical instrument having a portamento property and capable of continuously varying the tone pitch from a frequency corresponding to the note of a first key to that corresponding to the note of the a second, subsequently operated, key.
A typical prior art electronic musical instrument having the portamento property is disclosed in U.S. Pat. No. 3,866,836 issued June 3, 1975. The basic construction of an electronic musical instrument of this type will be described with reference to FIG. 1 of the accompanying drawings. When a key of a keyboard section 11 is depressed there are produced a voltage signal KV (hereinafter termed a pitch voltage ) corresponding to the pitch of the note of the operated key, and a pulse signal KS (hereinafter termed of keying signal) having a width corresponding to a period of time during which the key is depressed. The pitch voltage KV is applied to act as an oscillator driving signal to a voltage controlled type oscillator 17 (hereinafter called VCO) via voltage holding time constant circuit 12 comprising a switching element 13 in the form of a field effect transistor, a variable resistor 14, a capacitor 15 and a portamento property selection switch 16 connected in parallel with the variable resistor 14 for producing a tone source signal. The tone source signal is applied to a voltage controlled filter 18 (hereinafter termed VCF) to form a musical tone by coloring a tone. The tone signal produced by VCF 18 is subjected to the control of a musical tone level that is an envelope in a voltage controlled type variable gain amplifier 19 (hereinafter called VCA), and the output of this VCA is amplified by an amplifier 20 to produce a tone from a loudspeaker 21. The keying signal KS is applied as a driving signal to the voltage holding time constant circuit 12 which is used to hold the pitch voltage KV snd to impart the portamento property, and to control voltage generators 21, 22 and 23 (hereinafter termed CVG). In response to the keying signal KS generated by key, these control voltage generators CVG's generate time-variable control voltage signals controlled by a variety of parameters which are set in a parameter control voltage generator 24, and these control voltage signals are applied to VCO 17, VCF 18 and VCA 19 respectively. In the VCO 17, the oscillation frequency is finely varied in accordance with the control voltage signal from CVG 21, while in VCF 18, the cut-off frequency is varied to form a musical tone signal resembling a natural musical tone. The VCA 19 operates to form a musical tone envelope in accordance with a control wave signal. During the normal play the selection switch 16 of the voltage holding time constant circuit 12 is closed so as to apply the pitch voltage KV generated by a depressed key directly to VCO 17 via the selection switch 16 and to store the tone voltage KV in capacitor 15. The purpose of capacitor 15 is to hold the pitch voltage KV for obtaining a sustained tone after release of the key while the purpose of the switching element 13 is to prevent the discharge of the voltage held by the capacitor 15.
In an electronic musical instrument having the construction described above, where it is desired to provide a portamento property, the portamento property selection switch 16 is opened to charge the pitch voltage KV in capacitor 15 via variable resistor 14 so that the voltage applied to VCO 17 varies with a time constant determined by the variable resistor 14 and capacitor 15. A pitch voltage KV.sub.1 corresponding to a previously depressed key (the first key) is stored in the capacitor 15 as shown in FIG. 2 and when a new pitch voltage KV.sub.2 corresponding to a subsequently depressed key (the second key) is generated at time t.sub.1, the terminal voltage of the capacitor 15 increases logarithmically as shown by a solid line in FIG. 2 at a speed corresponding to the time constant determined by the variable resistor 14 and capacitor 15. As a consequence, the oscillation frequency of VCO 17 varies continuously as shown by the solid line in FIG. 2, whereby the pitch varies continuously from the pitch of the first key to that of the second key. When the audiences hear such musical sound having the portamento property, since the pitch frequency of the tone source signal produced by the VCO 17 varies rapidly and then slowly as shown by the solid line curve shown in FIG. 2, it varies differently from the actual pitch variation in the natural portamento shown by dotted lines in FIG. 2 thus giving an unnatural feeling to the audiences.